When high temperatures and high humidity combine, animals (as well as people) are in danger from heat stress, heat stroke, dehydration, and other over-heating problems. Any weaknesses these stressed animals have are brought out by these environmental stresses, allowing expression of underlying disease states as well.
Since dogs and cats (and some other pets) cannot sweat through skin pores (i.e., sweaty skin exposed to fans/breezes helps us cool our skin) to try to clear excess heat, they pant. Long-term and extensive panting can promote exhaustion, dehydration, and even increase body heat through the muscle activity (similar to shivering to produce heat in low temperature conditions). If the humidity is high and evaporation is less effective, heat disease can occur quickly. Especially in dogs and cats with short faces, small noses, and/or long soft palates (e.g., Bulldogs, Pugs, Persians, etc.), panting is often less effective at heat control and leaves these individuals prone to heat stroke much faster than others.
If your pets will be exposed to high temperatures and/or high humidity be sure that they are in shade, have access to sufficient clear, cool, fresh water, and have moving air (fans or breezes) if possible (moving air won’t reduce their body heat much but it will help them exchange some heat). If you do not have access to air conditioning, a home-made alternative is to blow a fan across a pan of ice cubes; the moving air cools and reduces humidity due to the drop in temperature providing a cooling drier breeze than without the apparatus.
When left outdoors, older, arthritic dogs sometimes lack the energy and effective desire to get out of the sun. This can often lead to heat stroke, coma, even death.
If your pet is panting heavily, frantic and uncomfortable, and possibly no longer making spit, call your veterinary office immediately. Get advice from the medical staff taking into account the breed, age, ambient temperature, and relative humidity in your area. You may be instructed to take a rectal temperature (normal is 100-102 degrees Fahrenheit); wet your pet all over with cool water; rinse their feet/groin/armpits and under their neck with rubbing alcohol; or combinations of the above. Do as recommended by your veterinary office when you call. You may be instructed to bring them as quickly and safely as possible to the hospital.
When heat stroke victims come in, most veterinarians run cool intravenous fluids, apply alcohol and cold water baths, monitor body temperature, and administer some shock medications. Unfortunately, the shock to the system often can't be reversed or controlled and many victims die.